Process for making resin for wet strength paper



Patented Apr. 7, 19 53 PROCESS FOR MAKING RESIN FOR WET STRENGTH PAPER Glenn A. Nesty and Harry E. Gronich, Morris. town, N. J assignors to Allied Chemical & Dye Corporation, New York, N. Y., a corporation of New York No Drawing.

Application October 6, 1951, Serial No. 250,193

6 Claims. (01. zen-29.4)

This invention relates to a process for the production ofan aqueous solution of a urea-formaldehyde condensation product of such character that it is selectively adsorbable from aqueous solution by cellulose fibers, and is particularly suitable for use in treating such fibers which are made into paper, to give a high wet strength to the paper. The invention is particularly directed also to the production of paper by treating the paper fibers with the urea-formaldehyde condensation products produced in accordance with the methods described herein. Our invention further extends to the solutions of condensation products produced by our novel process.

Processes for the production of water-soluble urea-formaldehyde condensation products or resins have heretofore been known. It has also been known that by treating slurries of cellulose fibers present in certain stages of the paper making process with solutions of certain of those water-soluble urea-formaldehyde condensation products or resins prepared under carefully controlled conditions, the resin is adsorbed by the cellulose fibers. By heating the paper made from the thus treated fibers to cure the adsorbed resin, both the dry and wet strengths of the paper are increased. The increase in wet strength of the paper i particularly important a inasmuch as paper, when wet, ordinarily retains only a very small fraction of its strength when dry. I

As is well known in reacting urea and formaldehyde in aqueous solutions to form condensation products thereof, depending upon the reaction co'ndition-s, water-soluble or crystalline or amorphous, insoluble products are produced. Most of these materials, including many of the -water-soluble products, are of no use for the cipitates which are unsatisfactory for the treat-.

ment of paper pulps. Thus, while the prior art describes a'multitude of procedures for the production of various condensation products of urea and formaldehyde, only a very few methods give water-soluble products which are usable for treatment of paper pulp and these require a close control of numerous reaction conditions during the reaction of the formaldehyde and urea.

The copending application of Harry E. Gronich, Serial No.l95,423, filed November 13, 1950, describes a process for the production of stable aqueous solutions of a urea-formaldehyde condensation product selectively adsorbable from acidic aqueous solution by cellulose fibers, which condensation products have properties making them particularly useful for the production of high wet strength papers. The process of that applicationinvolves:

1. Mixing and initially reacting urea and formaldehyde in an aqueous solution containing an acidic buffer salt and maintained at tem peratures in the ran 10C. to C. and at acidities in the range pH 4.5 to 6.9, the initial mol ratio of urea and formaldehyde in the reaction mixture being in the range 1.65 to 3 mols formaldehyde to one mol urea. The reaction mixture is maintained at 10 C. to 35 C. fora period of at. least one hour.

2. Thereafter, the liquid product of step 1 is heated to temperatures above C. and maintained at those'temperatures and at acidities in the range pH 4.5 to 6.5 until the'precipitation time of the reaction time test set forth in that application is substantially at its minimum value for the reaction mixture being treated. At this time the reaction mixture is cooled to atmospheric temperatures to arrest-the condensation reactions taking placein thi second step of the process. l

When a formaldehyde solution containing about 0.03% or more free formic acid is"employed as a starting material it is preferred to use as the buifer salt zinc formate incorporated in the formaldehyde solution by adding to the solution an excess of zinc dust over the amount required to react. with the formic acid to form zinc formate buffer salt. The presence of excess undissolved zinc in contact with the reaction mixture appears to promote formation of a resin product which imparts high wet strengths to paper made from cellulose fibers treated with that product. However, this excesszinc. should not be kept suspended in the reaction mixture. After s01id urea has been dissolved in the formaldehyde solution or after a solution of urea has been mixed with the aqueous formaldehyde, in stirring the reaction mixtureduring the precondensation and subsequent condensation reaction period, the rate of Stirring should be insufiicient to suspend unreacted finely divided zinc present in the reaction mixture. If more than minutes is required for dissolving the urea or mixing a concentrated urea solution with the formaldehyde solution, the stirring of the mixture should be carried out under conditions permitting zinc particles present to settle out of the reaction mixture within a period of 10 minutes from the initial addition of the urea to the formaldehyde solution. In other words, promptly after the urea has been dispersed in the formaldehyde solution or within a period of about 10 minutes after addition of urea to the formaldehyde solution, whichever period is shorter, agitation of the reaction mixture should be insumcient to maintain in suspension any zinc present. A slow stirring is generally desirable during the precondensation and during the following condensation period.

When the condensation step of the described process is carried out at or below the boiling point of the reaction mixture, the condensation reaction proceeds relatively slowly, requiring several hours or more to go to the stage at which the desired product is obtained.

The present invention is directed to an improvement in the process of said application Serial No. 195,423 of Harry E. Gronich. We have now discovered that if the second step, the condensation step, is carried out by quickly heating the reaction mixture to temperatures substantially above its boiling point at atmospheric pressure, and while thus highly heating the reaction mixture, maintaining it under pressures at least as high as those autogenously developed by the reaction mixture, and then quickly cooling the reaction mixture to room temperatures, the desired product 'may be obtained with short reaction periods in this step and without side reactions promoted'by the higher temperatures destroying or seriously impairing the effectiveness of the product for making high wet strength paper.

To attain these objectives in carrying out the process of our present invention, the reaction mixture being treated in the condensation step of the process described in the above-identified application of Harry E. Gronich is quickly heated from the temperatures of 10-35 C. at which it is maintained during the precondensat on step to temperatures above 100 C. but not above 165 0., preferably to temperatures above 120 C. but not above 150 C. and is quickly cooled to temperatures below 85 C. to arrest progress of the condensation reactions. The further cooling of the reaction product to atmospheric temperatures may be also carried out quickly or may be over a much longer time than that involved in the heating to the high'reaction temperature and cooling to below 85 C. It is best, however, that the cooling to atmospheric temperatures follow without prolonged delay the cooling to below 85 C. By quickly heating and quickly cooling the reaction mixture, we mean that it is heated to a temperature in the range above 100 C. to not above 165 C. and is cooled to not above 85 C. in 'a period no longer than 30 minutes, preferably is heated to 120 C. to 150 C. and cooled to not above 85 C. in no more than minutes.

The reaction mixture "need be at temperatures above 100 C. for only a short time for'the con densation reactions to go to the desired extent; Minimum times of 1 minute at temperatures above 120 C. and of 2 minutes at temperatures above 100 C. when the reaction mixture is heated no higher than 120 C;, are sufficient irrespective of the rapidity of heating the reaction mixture to above 100 C. and cooling it to not above C. These minimum times during which the reaction mixture is at temperatures above C. are included in the aforedescribed periods within which the reaction mixture is quickly heated and cooled. While being heated to the high temperatures above its boiling point, the reaction mixture is maintained under pressures at least as great as those autogenously developed at the temperatures to which the reaction mixture is heated to prevent loss from the reaction mixture of substantial amounts of reactants. Under the foregoing conditions we have found stable solutions of urea-formaldehyde reaction product are obtained which impart high wet strengths to paper.

Other than time, temperature and pressure for carrying out the second step, the reaction conditions of acidity and composition of reaction mixture, the conditions under which it is subjected to the first (precondensation) step and treatment of the product of the second (condensation) step prior to its storage or use in paper making, are the same as for the process described in said application of Harry E. Gronich.

In operating in accordance with our invention, the heating and cooling of the reaction mixture are carried out rapidly, in order both to suitably control the time of the reaction period (and thereby the degree of condensation of the ureaformaldehyde) and to minimize hydrolysis of the urea. Once the reaction mixture has been cooled to below 85 C., further condensation of the ureaformaldehyde takes place so'slowly that a relatively slow cooling to atmospheric temperatures may be employed.

Solutions of condensation product prepared by the processes of our invention are stable for long periods of time at atmospheric temperatures without gelation or deposition of solids therefrom and are thus suitable for production and marketing for use in paper manufacture. The stability of the solutions may be increased whenever desired by adding an alkali, such as sodium hydroxide, to neutralize the free acidity of the solution and to reduce its hydrogen ion concentration to a pH of '7 or slightly higher, i. e. 7-7.5.

The presence of methanol in the solution of reaction product prepared in any of the described manners further increases the storage life of the solution. About 2%% or more methanol by weight of the reaction product is sufiicient to materially increase the storage life. When methanol is to be present, we prefer the products contain 5% methanol and practical considerations of the cost of added methanol and dilution of the reaction product will limit the methanol addition to a minor amount, not greatly exceeding this preferred 5%. The commercial inhibited formalin solutions on the market usually contain about 6% to 13% methanol to serve as a preservative for the formaldehyde solution during shipment and storage. In general, this quantity of methanol in the formaldehyde solution used for making up the reaction mixture will result in the reaction product containing an adequate methanol concentration without the addition of more methanol to that product.

As described in said application of Harry E. Gronich, instead of neutralizing the product following the condensation step at high temperatures, it may be stored at atmospheric temperatures for a period of time to age it and to increase the Wet strengthof paper prepared from fibers treated with such aged solutions. By neutralizing the aged solutions their stability is increased and the time during whichv they. may.

be held further at atmospheric temperatures before use in paper production is substantially longer.

I Also, when aqueous solutions of formaldehyde containing major amounts of water such as the commercial 37% by weight formaldehyde solutions are employed in making up the reaction "mixture, it may be and frequently is desirable to eliminate at least a part of this water so that .thefinal product will contain a relatively high concentration of solids. acidity of the reaction product of the high tem-,

perature condensation stage of the process described above, the neutralized aqueous products and without substantially decreasing the wet strength imparted to paper by treatment with the concentrated product. However, when the product solution neutralized to 7.0-7.5 is evapo-i rated far enough for the pH of the solution to drift below 7.0 downwardly to 6.0, the desired quantity of water should be evaporated from the solution sufficiently rapidly so that the solution acidic by addition of an acid or acidic material such as the alum commonly added to the pulp slurries in paper making processes. Thus, the resin solution may be added tothe fiber suspension in the heater or head box commonly employed in paper manufacture. The amount of resin solution'used is determined by the properties to be imparted to the paper; more or less resin solution being used, depending upon eco- .nomics and the increase in wet or dry strength desired for the paper. The urea-formaldehyde condensate is preferentially adsorbed from solu Addition to the fiber .tion by the cellulose fibers.

By neutralizing the suspension of resin solution containing a frac- .tion of a percent up to 2 /2% by weight of solids content of the solution based on the dry weight of the cellulose fibers present, increases sharply v the wet strength of the resulting paper. In general, however, further increases above 2 in the amount of urea-formaldehyde resin added do. not give comparable increases in the wet strength of the paper product. In addition to the to the paper manufacturer has a hardness of this f nature, addition of a water softening agent, such fibers with the solutions of urea-formaldehyde resin. Instead of reducing the hardness of the as sodium hexa-metaphosphate, will obviate this reduction ineffectiveness of the treatment of the water, adding to it an acid or acidic material in amount sufficient to increase its hydrogen ion' concentration to a pH of about 4.0 to about 4.2 will also restore the effectiveness of the resin solution in increasing the wet strength of the paper.

The fiber pulp containing the adsorbed ureaj formaldehyde condensation product is sheeted by.

the usualimethods employed by paper manufacturers. The paper is dried and heated to cure the resin. The time required for adequate curing of the resin decreases as the temperature of heating is increased. Adequate rates of curing are obtained by heating the paper at temperatures in the range to 150 C.

The following examples more particularly illustrate our process for the production of solutions of urea-formaldehyde condensation products.

In employing the product solutions of these examples for making paper, Kraft pulp was beaten with water to form a slurry to which the resin cured at C. for 5 minutes under pressure.

Samples of paper thus prepared and paper prepared in the same manner except for no resin solution being added to the pulp slurry were conditioned 24 hours at 75 F. and 50% relative humidity and then tested for dry and wet tensile strengths. The tensile strength tests were made on a Scott I.P.-4, inclined plane constant rate of load machine, using wide samples both for wet and dry tests. The test data are given in termsof kg./gm. wt. of a strip of paper 10 cm. long and wide. For wet strength test sample strips were soaked 60 seconds in distilled water, then placed between two blotters. A steel cylinder 3" long weighing 3 kg. was rolled across the blotters and test strip under its own weight and the paper strip was tested immediately. l

For comparison with the values given in the following examples for the'wet strengths of papers made in the manner described, the wet tensile strengths of paper prepared in the same manner except that no urea-formaldehyde. reaction product was added to the pulp slurry, were 3%5 kg./gm. wt.

Ewample 1.-Into a vessel equipped with an agitator, 199 parts of 36.0% commercial formaldehyde solution containing 10-11% methanol and .03% formic acid was charged. To this was added .085 part zinc dust, and the solution was stronglyagitated until the pH increased to 6.15. There was then added 57.3 parts of solid urea, with strong agitation to obtain rapid solution of the urea. This corresponded to a ratio of 2.5 mols formaldehyde for 1 mol urea. When the urea had dissolved the agitation was slowed down and the reaction mixture was stirred slowly at 20-25 0. (room temperature) for three hours. Following the precondensation period the reaction mixture was transferred into a bomb, sealed, then immersed in an oil bath preheated to 220 C. After 12% minutes in the oilbath, the bomb was removed and immediately quenchedin -.a.co1d water bath. During the period in the hot oil the temperature of the bomb contents rose from about 25 C. to C. with development of 90 p. s. i. gaugepressureina period of 12 /2 m n- :utes. In .the cold waterbath the bomb contents were-cooled to below 85 C. in about 4 minutes and to -25 C. in 15 minutes.

The solution of urea-formaldehyde condensation product thus prepared is clear and mobile and has a pH of about 4.6. It is stable when stored at atmospheric temperatures over long periods of time even without being neutralized. 'When this solution was employed in making paper by the procedure described above, the paper had a Wet strength of kg/gm. wt.

Example 2'.'-The same procedure described in Example 1 was carried out except that the bomb containing the reaction mixture was immersed in the oil bath for 20 minutes before being Withdrawn and cooled in the cold water bath. In this case the temperature in the bomb rose from 24to a maximum of 165 C. during the period of 20 minutes immersion in the oil bath. At the end of the 20 minute heating up period the pressure in the bomb was 325 p. s. 1. gauge. In the water bath the bomb contents Were cooled to a temperature below 05 C. in a period of less than 10 minutes.

The product thus obtained had a pH of-5.9 and contained some White precipitate which readily settled out. The clear, liquid solution of ureaformaldehyde condensation product was stable at atmospheric temperatures over a long period of time and paper prepared with this resin in the manner described above had a wet strength of 11.3 lie/gm. wt. I

Employing the same procedure except for iminersion of the bomb containing the reaction mixture in an oil bath for 12 minutes during which period the temperature of the bomb contents rose from 26 C. to 153 C., followed by immersion of the bomb in the cold water bath, a stable solu tion of urea-formaldehyde reaction product was obtained having a pH of i.8. This product imparted to paper prepared in the manner described above a wet strength of 18.6 kg. gm. wt.

We claim: v

1. In a process for the production of a stable, aqueous solution of a urea-formaldehyde condensate selectively adsorbable fromacidic aqueous solution by cellulose fibers which comprises 1) maintaining for a period of at least one hour at temperatures in the range 10 C. to 35 C. an aqueous solution initially containing formaldehyde and urea in the mol ratio of 1.65 to 3 mols formaldehyde for every 1 mol urea and an acidic buffer salt and maintaining the hydrogen ion concentration of said solution within the range 111-145 to 6.9, said solution having been prepared by mixing the urea with the formaldehyde in an aqueous solution having a pH within the aforesaid range; (2) then heating the thustreated re action mixture to temperatures above 50 C. and maintaining it at acidities in the range pH- 4.5 to 6.5 to cause further condensation of the urea and formaldehyde; and (3) cooling the thus heated liquid reaction mixture to atmospheric temperatures and thus arresting the condensation reactions; the improvement which comprises in step 2 quickly heating the reaction mixture to temperatures in the range above 100 C. to no higher than 165 C. under pressures at least as great as those autogenously developed at those temperatures and in step 3 quickly cooling the thus heated reaction mixture to a temperature no higher than 85 C., said heating and cooling of the reaction mixture being accomplished in a period no longer than 30 minutes, including a minimum of 1 minute during which the reaction mixture isiat't'emperatures, above 120 C. and

when the reactionrmixture is heated to no higher than 120 C., a minimum of 2 minutes during which it is at temperatures above 100 C.

2. The process of claim 1 wherein in step 2 the reaction mixture is quickly heated to temperatures in the range above 120 C. to 150 C., and in steps 2 and 3 the heating and cooling of the reaction mixture is accomplished in a period no longer than 20 minutes, including-a minimum of 1 minute during which the reaction mixture is at temperatures above 120 C.

3. In a process for the production of a stable, aqueous solution of a urea-formaldehyde condensate selectively adsorbable from acidic aqueous solution by cellulose fibers which comprises ('1) mixing with an aqueous formaldehyde solution containing at least 0.03% free formic acid, zinc dust in amount in excess of that required to formzinc formate with the free formic acid present,agitating the mixture of solution and zinc dust until the zinc hasreacted with a substantial proportion of the formic acid and the pH of the solution is above 4.5; (2) thereafter adding to and dispersing in the formaldehyde solution urea in amount providing 1 mol of urea for every 1.6.5 to 3 mols formaldehyde present, promptly after dispersal of the urea in the formaldehyde solution and within 10 minutes of the initial addition of the urea to the solution, separating out from suspension in the solution undissolved zinc suspended therein; (3) maintaining the thus prepared reaction mixture for a period of at least one hour at temperatures in the range 10 C. to 35 C. and at hydrogen ion concentrations in the range pH i=5 to 6.9; (4) then heating the thus treated reaction mixture to temperatures above 50 C. and maintaining it at acidities in the range pH 4.5 to 6.5 to cause further condensation of the urea and formaldehyde; and (5) cooling the thus heated liquid reaction mixture to atmospheric temperaturesancl thus arresting the condensation reactions; the improvement which comprises in step 4'quickly heating the reaction mixture to temperatures in the range above 100 C. to no higher than 165 C. under pressures at least as great as those autogenously developed at those temperatures and instep 5- quickly cooling the thus heated reaction mixture to a temperature no higher than C., said heating and cooling of the reaction mixture-being accomplished in a period no longer than 30 minutes, including a minimum of 1 minute during which the reaction mixture is at temperatures above 120 C., and when the reaction mixture is heated to no higher than 120 C., a minimum of 2 minutes during which it is at temperatures above C. r e. The process of claim 3 wherein 1 mol of urea for every 2 to 3 mols of formaldehyde are added instep 2, in step 4 the liquid product of Step '3 is heated to temperatures in the range above C. to C., and in steps 4 and 5 the heating and cooling of the reaction mixture are accomplished in a period no longer than 20 minutes, including a minimum of'l minute during which the reaction mixture is at temperatures above 120 C. I 5. A stable aqueous solution ofa urea-formaldehyde condensation product selectively adsorbablefrom acidic aqueous solution by cellulose fibers," which condensate product has been produced by the process of claim 1.

6. A stable aqueous solution of a urea-formaldehyde condensation product selectively adsorbable from acidic aqueous solution by cellulose fibers which cend'ensate product has been pro- REFERENCES CITED The following references are of record in the file of this patent:

meme-r UNITED STATES PATENTS Number Name Date 2,338,602 Schur Jan. 4, 1944 2,559,220 Maxwell et a1. July 3, 1951 FOREIGN PATENTS Number Country Date 467,505 Canada Aug. 22, 1950 

1. IN A PROCESS FOR THE PRODUCTION OF A STABLE, AQUEOUS SOLUTION OF A UREA-FORMALDEHYDE CONDENSATE SELECTIVELY ADSORBABLE FROM ACIDIC AQUEOUS SOLUTION BY CELLULOSE FIBERS WHICH COMPRISES (1) MAINTAINING FOR A PERIOD OF AT LEAST ONE HOUR AT TEMPERATURES IN THE RANGE 10* C. TO 35* C. AN AQUEOUS SOLUTION INITIALLY CONTAINING FORMALDEHYDE AND UREA IN THE MOL RATIO OF 1.65 TO 3 MOLS FORMALDEHYDE FOR EVERY 1 MOL UREA AND AN ACIDIC BUFFER SALT AND MAINTAINING THE HYDROGEN ION CONCENTRATION OF SAID SOLUTION HAVING BEEN PREPARED PH 4.5 TO 6.9, SAID SOLUTION HAVING WITHIN THE RANGE BY MIXING THE UREA WITH THE FORMALDEHYDE IN AN AQUEOUS SOLUTION HAVING A PH WITHIN THE AFORESAID RANGE; (2) THEN HEATING THE THUS TREATED REACTION MIXTURE TO TEMPERATURES ABOVE 50* C. AND MAINTAINING IT AT ACIDITIES IN THE RANGE PH 4.5 TO 6.5 TO CAUSE FURTHER CONDENSATION OF THE UREA AND FORMALDEHYDE; AND (3) COOLING THE THUS HEATED LIQUID REACTION MIXTURE OF ATMOSPHERIC TEMPERATURES AND THUS ARRESTING THE CONDENSATION REACTIONS; THE IMPROVEMENT WHICH COMPRISES IN STEP 2 QUICKLY HEATING THE REACTION MIXTURE TO TEMPERATUERS IN THE RANGE ABOVE 100* C. TO NO HIGHER THAN 165* CC. UNDER PRESSURES AT LEAST AS GREAT AS THOSE AUTOGENOUSLY DEVELOPED AT THOSE TEMPERATURE AND IN STEP 3 QUICKLY COOLING THE THUS HEATED REACTION MIXTURE TO A TEMPERATURE NO HIGHER THAN 85* C., SAID HEATING AND COOLING OF THE REACTION MIXTURE BEING ACCOMPLISHED IN A PERIOD NO LONGER THAN 30 MINUTES, INCLUDING A MINIMUM OF 1 MINUTE DURING WHICH THE REACTION MIXTURE IS AT TEMPERATURES ABOVE 120* C. AND WHEN THE REACTION MIXTURE IS HEATED TO NO HIGHER THAN 120* C., A MINIMUM OF 2 MINUTES DURING WHICH IT IS AT TEMPERATURES ABOVE 100* C. 